The present invention is described below with respect to a liquid crystal shutter. It will be appreciated, though, that features of the invention may be utilized with shutters formed of materials other than liquid crystal and also may be utilized with devices other than shutters. A shutter as used herein refers to a device for controlling the transmission of electromagnetic energy or electromagnetic radiation, for example, being transmitted or not through the shutter. In the preferred embodiment described in detail below, such electromagnetic energy is in the form of light and more preferably is in the form of electromagnetic energy that is in the visible spectrum as well as in the various infrared spectra and ultraviolet spectra.
An exemplary liquid crystal shutter with which the driving circuit of the invention may be utilized is disclosed in U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, and Re. 32,521. An example of such shutter includes a pair of linear polarizers, one being used as an input polarizer and the other as an output analyzer, and a variable liquid crystal optical retarder between the two polarizers. By changing the electric field applied to liquid crystal in the retarder, the plane of polarization of the light transmitted through the retarder can be changed; and the intensity of light transmitted through the analyzer will be a function of the polarization direction of the light transmitted through the retarder.
One approach to providing for high speed operation, for example, in the microsecond range, say from several microseconds to several tens of microseconds, for such optical shutter, operation is according to surface mode effect, whereby that liquid crystal material located near the center of the liquid crystal cell along the optical transmission direction through the cell is maintained in a preferred alignment during cell operation, whereas liquid crystal material nearer the respective surfaces of the cell, i.e., the interface between the liquid crystal material and the respective glass plates, is switched as a function of the applied electric field, as is described in detail in the above-mentioned patents. In one embodiment some means, such as a bias voltage or electric field, or some other means or mechanism, is used to obtain the preferred alignment of liquid crystal material near the center of the cell. Application of a larger voltage/electric field compared to the exemplary bias voltage/electric field, effects switching of the alignment of the liquid crystal material nearer the surfaces.
The exemplary shutter may be used in a variety of embodiments and applications. One example is as a lens or shutter for a welding helmet. The shutter may be used to increase or to decrease the amount of light transmitted through the shutter. Therefore, when welding is not occurring, the shutter may be substantially optically transmissive. When welding is occurring, the shutter may be closed to minimize the amount of light transmitted therethrough in order to protect the eyes of the person performing the welding. A photosensitive device may be used to sense the intensity of light impinging in the area of the shutter so as to provide an input to a drive circuit for the shutter in order to control opening and closing thereof.
A problem experienced in the past has been the time required for a drive circuit to energize a variable liquid crystal retarder in the above-described shutter. Using such shutters in a welding helmet and in other environments where there is not a convenient access to a power supply connection directly to a utility company, battery power ordinarily must be used to drive the shutter. The problem encountered with prior driving circuits for such shutters has been the power drain on the battery or other portable power supply.